Hand-held electronic sensing devices are widely used to detect the presence of guns, knives, bombs and the like concealed on the persons of airport passengers and prison visitors. The level of sensitivity must be low enough to detect a relatively small but dangerous item, but not so low as to be activated by items too small to be likely to cause trouble, such as metal eyelits for shoe laces. The minimum level for giving a positive signal varies with particular applications. A prison, for example, is concerned about items as small as knives and saw blades, whereas an airport is not apt to be concerned about blades, but does need a signal for the presence of a small gun, for example.
Maintaining each sensing device at a predetermined standard level of sensitivity has presented a serious problem. The performance of a metal detector can vary significantly due to changes in ambient temperature or electromagnetic conditions, loss of battery strength, or adverse effects of shock, moisture or age on other components. The device usually has controls for adjustment to restore performance, but test runs with commonly available test objects, such as a screwdriver, lack precision, uniformity and reliability of the data obtained. Of course, the devices can be sent to a central calibration facility, but that causes delay and expense.